swales filled with fractured rocks

[chicopee]

One local ski area has developed swales in their dirt parking lot. These swales are filled with fractures rocks around 6" to 8" at their longest dimensions. The surfaces of the rocks are even with the swale surfaces. Swales are about 5' wide at surface and perhaps 1' deep. Which equation would be appropriate to calculate drainage flow. I am thinking of Manning's equation with an appropriate roughness factor but looking at these swales, we do not have open channels. Please comment.

 

[Ryb01]

Your intuition is correct to caculate the flow in the swale. a mannings equation would probably be your best bet. However, the cushed rock produces an interesting scenario. The simplest solution may be to calculate the flow in the swale using the mannings equation and nitially assume the rock is not in the swale. Next, mutliply your flow by Say a 40% stone void within your channel, (I.e 60% of the total flow area would be blocked with stone).

This is an over simplified way to estimate a flow. Due to the random placement, varying stone size, embedment depths, etc it would be near impossible to estimate a hydraulic radius within the channel and or consistent flow area between the stones. Also, flow chracteristics would change quite frequently between critical and subcrtical.

Hopefully this helps.

 

[chicopee]

Ryb01, thanks for your input. Your method is logical and I'll proceed in that direction.

 

[cvg]

invert of your channel should be assumed to be at the top of the rock surface. Any flow between, under and through the rocks is mostly ineffective at high discharges since the velocity is so low.

 

[dicksewerrat]

You are probably looking at overland sheet flows. The swale won't get rid of much water.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com